Kotlin: ‘This type has a constructor and thus must be initialized here’, but no constructor is declared

constructorinitializationkotlinoopsuperclass

Recently started with Kotlin

According to Kotlin docs, there can be one primary constructor and one or more secondary constructor.

I don't understand why I see this error

Since class test has no primary constructor.

This works fine:

open class test {
}

class test2 : test() {
}

And here is another difficulty I have faced, when I define a secondary constructor the IDE shows another error saying

Supertype initialization is impossible without primary constructor

But in the previous working example, I did initialize it, yet it worked fine. What did I get wrong?

Best Answer

You get this error because, even if you don't define a primary or a secondary constructor in a base class, there is still a default no-argument constructor generated for that class. The constructor of a derived class should always call some of the super constructors, and in your case there is only the default one (this is the constructor that you can call like test() to create an object of the class). The compiler and IDE force you to do that.

The super constructor rules complicate the matter to some degree.

If you define a secondary constructor in the derived class without defining the primary constructor (no parentheses near the class declaration), then the secondary constructor itself should call the super constructor, and no super constructor arguments should be specified in the class declaration:

class test2 : test { // no arguments for `test` here
    constructor(a: Int) : super() { /* ... */ }
}

Another option is define the primary constructor and call it from the secondary one:

class test2() : test() {
    constructor(a: Int) : this() { /* ... */ }
}

Related Solutions

Java – Why do this() and super() have to be the first statement in a constructor

The parent class' constructor needs to be called before the subclass' constructor. This will ensure that if you call any methods on the parent class in your constructor, the parent class has already been set up correctly.

What you are trying to do, pass args to the super constructor is perfectly legal, you just need to construct those args inline as you are doing, or pass them in to your constructor and then pass them to super:

public MySubClassB extends MyClass {
        public MySubClassB(Object[] myArray) {
                super(myArray);
        }
}

If the compiler did not enforce this you could do this:

public MySubClassB extends MyClass {
        public MySubClassB(Object[] myArray) {
                someMethodOnSuper(); //ERROR super not yet constructed
                super(myArray);
        }
}

In cases where a parent class has a default constructor the call to super is inserted for you automatically by the compiler. Since every class in Java inherits from Object, objects constructor must be called somehow and it must be executed first. The automatic insertion of super() by the compiler allows this. Enforcing super to appear first, enforces that constructor bodies are executed in the correct order which would be: Object -> Parent -> Child -> ChildOfChild -> SoOnSoForth

R – Browser feature detection: spell checking

You already know which browsers have built-in support and which browsers don't so you may use some form of browser sniffing to decide whether you enable the spell-checking component or not.

You may also try to ask your users if they already have some spell-checking enabled and let them answer Yes/No/Don't know. If they don't know, fall back to automatic detection. This is better than using sniffing only because sniffing is known to be unreliable in some circumstances.

Detecting things that are part of a browser's UI is hard, if possible. Due to browsers' security policies, a web site can't access most part of the API that could expose something useful for feature detection. And even if security was not a problem, you would probably still face one distinct API for every browser, since internal browser mechanics are not standardized.

Best Answer

Related Solutions

Java – Why do this() and super() have to be the first statement in a constructor

R – Browser feature detection: spell checking

Related Topic